1. Field of the Invention
The invention relates to a power converting device, more particularly to a DC-to-AC power converting device.
2. Description of the Related Art
FIG. 1 illustrates a conventional solar power converting system that includes a solar cell array 1 for converting solar energy into electrical energy, a power conditioner 2 for receiving the electrical energy from the solar cell array 1 and for outputting an AC voltage output to a load or a power network, and a rechargeable battery 6. The power conditioner 2 includes a charge/discharge controller 21 coupled to the solar cell array 1 and the rechargeable battery 6, a DC-to-DC converter 22 coupled to the charge/discharge controller 21, and a DC-to-AC inverter 23.
FIG. 2 illustrates a conventional boost device that can be applied to the aforesaid DC-to-DC converter 22 of FIG. 1. The conventional boost device includes an inductor 11, a switch 12, a diode 13, and a capacitor 14. The inductor 11 has a first end coupled to an external power source 10, and a second end. The switch 12, such as a semiconductor power switch, has a first terminal coupled to a common node between the second end of the inductor 11 and an anode of the diode 13, a second terminal coupled to ground, and a control terminal for receiving an external control signal such that the switch 12 is operable between an ON-state and an OFF-state in response to the external control signal. The capacitor 14 is coupled between a cathode of the diode 13 and ground.
When the switch 12 is operated in the ON-state, a current (iL) from the external power source 10 flows through the inductor 11 to store electric power. When the switch 12 is operated in the OFF-state, the capacitor 14 is charged with a current from the inductor 11 through the diode 13 such that the conventional boost device outputs an output voltage, i.e., a voltage across the capacitor 14, to a load.
The following are some of the drawbacks of the conventional boost device:
1. When the switch 12 is in the OFF-state, a voltage across the switch 12 is substantially equal to the output voltage. Therefore, if the switch 12 is implemented as a MOSFET device, a relatively large conducting impedance is exhibited by the MOSFET device, thereby resulting in a relatively large conduction loss.
2. When the switch 12 is switched from the OFF-state to the ON-state, a reverse bias surge current is generated to flow through the switch 12 that causes serious switching loss, thereby reducing power transformation efficiency.
FIG. 3 illustrates another conventional boost device that can be applied to the aforesaid DC-to-DC converter 22 of FIG. 1. The conventional boost device includes a coupling inductor 15, a switch 16, a diode 17, and an output capacitor 18. The coupling inductor 15 has first and second windings 151, 152, each of which has a polarity end and a non-polarity end. The polarity end of the first winding 151 is coupled to an external power source 10. The switch 16 has a first terminal coupled to a common node between the non-polarity end of the first winding 151 and the polarity end of the second winding 152, a second terminal coupled to ground, and a control terminal for receiving an external control signal such that the switch 16 is operable between an ON-state and an OFF-state in response to the external control signal. The diode 17 has an anode coupled to the non-polarity end of the second winding 152, and a cathode. The capacitor 18 is coupled between the cathode of the diode 17 and ground.
When the switch 16 is operated in the ON-state, a current from the external power source 10 flows through the first winding 151 such that the first winding 151 is excited to store electric power. When the switch 16 is operated in the OFF-state, energy stored in the coupling inductor 15 charges the output capacitor 18 through the second winding 152 and the diode 17 such that the conventional boost device outputs an output voltage, i.e., a voltage across the output capacitor 18, to a load.
When the switch 16 is switched from the ON-state to the OFF-state, a voltage is generated as a result of a leakage inductance of the coupling inductor 15 and can cause damage to the switch 16. As such, an additional snubber circuit is required to absorb energy attributed to the leakage inductance.
Since the operation of the conventional boost device is described in detail in the aforesaid patent, further discussion of the same is omitted herein for the sake of brevity.
However, such a conventional boost device cannot provide electrical isolation. Thus, for an outdoor power supplying appliance including the conventional boost device, lightning strike may result in damage to the conventional boost device.